On the Number of Water Molecules Necessary To Stabilize the Glycine Zwitterion

A thorough ab initio study of how the addition of successive water molecules shifts the gas phase zwitterion-neutral equilibrium of the amino acid glycine toward that of the solution phase is presented. Of particular interest is the number of water molecules necessary to stabilize the zwitterion, and how the solvent effects conformational preference. It is found that two water molecules can stabilize the glycine zwitterion, that is, give rise to a potential energy minimum with at least one vibrational level. The results are analyzed and explained using localized charge distributions. 35 refs., 11 figs., 4 tabs.